Various configurations are known for a driver circuit to drive a laser diode (which will hereinafter be referred to as an LD) for modulating intensity of an optical signal emitted by the LD. For example, Patent Literature 1 (Japanese Unexamined Patent Publication No. 2005-33019) discloses a shunt driver circuit which includes a transistor, a drain of which is connected to an anode of an LD, a source of which is connected to a cathode of the LD, and a gate of which is supplied with a modulation signal. Such a driver circuit outputs a shunt current, which is bypassed (shunted) from the bias current to generate a drive current of the LD in accordance with a modulation signal, so that the intensity of an optical signal emitted from the LD is modulated by the drive current. In addition, Patent Literature 2 (Japanese Unexamined Patent Publication No. 2007-266493) also discloses a similar configuration of a driver circuit.
In the foregoing shunt driver circuit in the related art, supplying of a bias current to an LD is sometimes stopped in order to shut off an optical signal. Supply stop of a bias current disables laser oscillation, so that the LD stops emitting an optical signal. In this case, in a transistor of a driver circuit, there are cases in which useless currents flow from a gate (base) toward a semiconductor substrate or a source (emitter). Therefore, power consumption tends to be ineffectively increased in a driver circuit. It is preferable that such useless power consumption be reduced.